1. Field of the Invention
The present invention relates to methods for producing porous tricalcium phosphate net-shaped materials comprising alpha or beta tricalcium phosphate or mixtures thereof by a combustion synthesis method.
2. Description of the Prior Art
The need for biomaterials in orthopedic and dental applications has increased as the world population ages. A significant amount of research into biomaterials for orthopedic and dental uses has attempted to address the functional criteria for orthopedic and dental reconstruction within the human body. The materials which have become available for such uses have improved in recent years. All such materials must be biocompatible, however, and the degree of biocompatibility exhibited by materials which are candidates for such use is always a major concern. Biomaterials useful for orthopedic and dental reconstructions must have high strength, must be able to be immediately affixed to the situs for reconstruction, must bond strongly to bone, and must give rise to strong, highly resilient restorations.
Tricalcium phosphate (TCP) materials are considered as one of the most preferred material types in the filed of orthopedic, restorative and reconstructive surgery, and are particularly useful for bone replacement, spinal repair, reconstructive, cosmetic and other surgeries. Tricalcium phosphate occurs in at least two forms. The first is the monoclinic form, called alpha tricalcium phosphate. The second form is the orthorhombic form, called beta tricalcium phosphate. Beta tricalcium phosphate (beta TCP) is the preferred form for bone replacements because it is capable of being resorbed by the body, facilitating bone remodeling. At appropriate porosities, beta TCP resembles natural bone and provides a scaffold for in-migration of osteogenic cells, resulting in production of bone directly attached to the beta TCP implant. The body will generally resorb beta-TCP within about two years, replacing it with natural bone.
Calcium phosphate ceramics have been fabricated and implanted in mammals in many different forms including as shaped bodies, in cements and otherwise. Different stoichiometric compositions such as hydroxyapatite (HA), tricalcium phosphate (TCP), and tetracalcium phosphate (TTCP), have all been employed to this end in an attempt to match the adaptability, biocompatibility structure and strength of natural bone. However, these ceramic biomaterials exhibited problems derived from chemical and processing shortcomings that limited stoichiometric control, crystal morphology, surface properties, and, ultimately, reactivity in the body. Intensive milling and comminution of natural minerals of varying composition was required, followed by powder blending and ceramic processing at high temperatures to synthesize new phases for use in vivo. Thus, despite tremendous efforts directed to the preparation of improved calcium phosphate and precursor hydroxyapatite materials for such uses, significant shortcomings still remain.
Current processes for preparing beta TCP implants have not yet been perfected. Two approaches have been taken toward the goal of producing TCP ceramics for use as bone replacements. The first approach has been by synthesis from aqueous solutions for use in bioceramic applications. The second approach involves sintering or sol-gel routes. The sintering process includes molding a powder to a required shape by a compacting process, then heating to a high temperature so that the particles may bond by solid-state bonding but not melt. However, this approach is undesirable for the production of beta TCP ceramics, since at high temperatures beta TCP is converted to alpha TCP, which is not preferable for a bone replacement material.
Several patents describe the preparation of porous inorganic or ceramic structures using polymeric foams impregnated with a slurry of preformed ceramic particles. The prior art also describes the use of solution impregnated-polymeric foams to produce porous ceramic articles. The focus of this art is directed to the preparation of either metal or metal oxide foams and/or particles.
However, the above-described technologies are highly energy- and labor-intensive, involving several discrete time-consuming operations. Thus, while improvements have been made in ceramic processing technology leading to tricalcium phosphate biomaterials, improved preparative methods are still greatly desired.